Computing digital averaging phase meter

ABSTRACT

In a ranging system, a first modulating signal is detected as an indication of the &#39;&#39;&#39;&#39;fine&#39;&#39;&#39;&#39; range from the transmitter to the target. A second modulating signal, a combined signal including &#39;&#39;&#39;&#39;fine&#39;&#39;&#39;&#39; range signal, and a &#39;&#39;&#39;&#39;coarse&#39;&#39;&#39;&#39; range signal is also detected. A digital phase meter sequentially measures the phase of the &#39;&#39;&#39;&#39;fine&#39;&#39;&#39;&#39; modulating signal and the combined signal, referred to a common reference. The results are simultaneously algebraically combined to obtain an indication of the &#39;&#39;&#39;&#39;fine&#39;&#39;&#39;&#39; range and the &#39;&#39;&#39;&#39;coarse&#39;&#39;&#39;&#39; range.

0 United States Patent 11 1 [111 3,766,555

Watt Oct. 16, 1973 [5 COMPUTING DIGITAL AVERAGING 3,360,797 12/1967Picou 343/14 PHASE METER 3,521,283 7/1970 Angelle 343/14 3,728,0254/1973 Madigan et al. 343/14 [75] Inventor: Richard E. Watt, San Diego,Calif. [73] Assignee: Cubic Corporation, San Diego, Primary Examiner-Benjamin Bofchelt C lif Assistant ExaminerG. E. Montone Attorney-MarvinH. Kleinberg [22] Filed: Sept. 28, 1972 [21] Appl. No.: 293,106 [57]ABSTRACT 7 In a ranging system, a first modulating signal is de- 5211.5. C1 343/14, 324/83 R, 324/186, tected as an indication of the rangefrom the 343 5 p transmitter to the target. A second modulating signal,511 1m. 01. G015 9/24, GOlr 25/00 a combined Signalinc1udingfinerangesignal, and a [58] Field of Search 343/14, 12 R, 5 DP; range Signal isalso detected- A digital phase 324 3 D, 3 R, 3 meter sequentiallymeasures the phase of the fine modulating signal and the combinedsignal, referred to 5 References Cited a common reference. The resultsare simultaneously UNITED STATES PATENTS algebraically combined toobtain an indication of the fine range and the coarse range. 3,315,2534/1967 Geller 324/83 D 3,223,998 12/1965 Hose 343/5 DP 9 Claims, 2Drawing Figures Down Counter C oc Clock Accumulator Generator Counter 32sotgce Data Signals l6 8 Up Counter 24 R Process Control Counter Sourceof Range Phase Output Ref. Signal 20 A Control Logic Process CommandPATENTEUUBT 16 I973 SHEET 10F 2 39:0 395 omcom UCOEEOO mmmoohl m mmLQFZJOO 5300 COMPUTING DIGITAL AVERAGING PHASE METER BACKGROUND OF THEINVENTION 1. Field of the Invention This invention relates to digitalphase meters and, more particularly, to a phase meter for performingsimultaneous algebraic operations.

The phase between two corresponding signals may be defined as the timeor distance measured in degrees between corresponding portions of twodifferent signals having the same frequency. The phase differencebetween a pair of sine waves having the same frequency can be measuredbetween corresponding portions when the individual sine waves cross thezero axis.

By considering either square waves or sine waves having the samefrequency, the phase can usually be measured between correspondingportions where the individual waves cross the zero axis. For example,the distance between the positive transition of a first wave crossingthe zero axis and the positive transition of a second wave crossing thezero axis is a measure of the phase difference between the two waves.Although difference is usually measured in degrees, in a digital timesystem, the phase difference can be measured in terms of an accumulatedcount of discrete pulses generated by a suitable clock generator.

2. Description of the Prior Art It is well known that the phasedifference between a pair of corresponding signals may be measuredeither by the distance between positive transitions of the two signalsor by measuring the distance between negative transitions of the twosignals. Both signals are usually symmetrical, and hence both readingsor distances should be the same. In a copending application, Ser. No.293108, filed Sept. 28, 1972, entitled SYM- METRICAL WAVE DIGITAL PHASEMEASURING SYSTEM of the present inventor and assigned to the sameassignee, a phase measuring system is described in which the individualsignals are not symmetrical. The teachings of the referred to patentapplication may be used in the present patent application should themeasured signals be found to be not symmetrical.

A second copending Pat. application, Ser. No. 293107, filed Sept. 28,1972, entitled AMBIGUITY FREE DIGITAL AVERAGING PHASE METER of thepresent inventor, and assigned to the same assignee, describes a systemfor eliminating the ambiguities in measuring the phase between twosignals, when the reference signal and the data signal are in phase witheach other. Provision is made for eliminating the possibility of makingreadings in the vicinity of zero phase difference by adding 360 to thesignals in question.

Phase measurements between and 180 are, in reality, measured between 360and 540 by adding 360 to the signals being measured. Phase differencesbetween 180 and 360 are measured in the conventional manner. Both thereference signal and the data signal are divided in frequency, and thephase difference between the divided signals are compared. The datasignal is inhibited for only 180 of the original reference signal toprevent initial comparing of phase differences between the data signaland the reference signal and to thereby establish the proper polarityrelationship between the divided data signal and the divided referencesignal. The teachings of the referred to patent application may be usedin the present patent application to eliminate the possibility ofmeasurement ambiguity in the presence of zero" phase difference betweenthe measured signals.

Phase measuring systems have found wide applicability in angle measuringsystems and distance measuring systems. So-called Loran" navigationsystems transmit a pair of accurately located signals in which themeasured phase or time difference between the signals is an indicationof present location on the Earth. The navigator receives and detects thepair of signals and, by means of suitable digital phase measuringequipment, accurately measures the phase or the time between the pair ofsignals. The phase difference can then represent his position on theface of the Earth. Accurate phase measuring equipment is thereforenecessary to determine precise location as a function of time.

Distance measuring systems of the so-called continuous wave type (cwRadar) continuously compare the phase of a transmitted cw signal with areceived cw signal as a measure of the distance of the object from thepoint of transmission The phase difference between the transmitted waveand the received wave can be related to the actual distance of theobject from the transmitter. Ranging systems utilizing digital phasemeters are fully described in U.S. Pat. Nos. 3,078,460 and 3,300,680,which are assigned to the same assignee as the present invention.

In continuous wave digital ranging systems, it is the common practice totake repetitive readings over a period of time in order to normalize thetotal reading and reduce the effects of external signals that may begenerated as a result of noise or other outside phenomena. A typicalranging system will have a carrier signal, for example, at 400 MHz andmay use a plurality of modulating frequencies to obtain ranginginformation. In one system fine range measurements are obtained by usinga 15 MHZ modulating frequency, and coarse range measurements areobtained by using a 1.50 MHz modulation frequency. In the usual case,each ranging frequency is used as a modulating signal to obtain thedescribed fine and coarse range measurements. Coherency of informationis obtained by first modulating the 15.0 MHZ fine frequency signal onthe carrier signal and measuring the phase between the returneddemodulated fine frequency signaland a locally generated referencesignal. Thecoarse range is obtained by modulating the 1.5 MHz signal onthe carrier and measuring the phase between the transmitted and receivedsignal to obtain the coarse range.

It will be apparent to those skilled in the art that many intermediaterange frequencies may be used. For a two-tone system just described, theband width required is 2 X 13.5 or 27 MHz. If a very coarse range of KHzis added, the bandwidth required becomes 2 X 14.85 or 29.7 MHz.

In an effort to reduce the bandwidth required, a system sometimes calledfolding the modulation frequency is used. In the folding system, thefine frequency of 15 MHz is used as before, however the coarse frequencyof 1.5 MHz is combined with it by either adding or subtracting. Theresult, if subtracting, of 13.5 MHz is used to modulate the carrier. Thebandwidth required is now 2 X l.5 MHz or 3 .0 MHz. By adding a verycoarse frequency of 150 KHZ, the bandwidth for the folded system is notincreased. However, the bandwidth for the unfolded system is increasedto 2 X 14.85 or 29.7 MHz.

In addition, the combining of frequencies can be repeated, dependingupon the actual resolution required by the system. For example, in somesystems it may be only necessary to combine twice and generate only twomodulating signals to obtain the accuracy desired. As many as fourcombined signals have been generated by combinging the intermediatefrequency with the fine, combining the coarse with the fine, and thecombining the very coarse with the fine, and sequentially modulating allthese combined signals on the carrier signal.

SUMMARY OF THE INVENTION The problem solved by the present invention isthe simultaneous measuring of the phase of the returned signal, togetherwith the computation necessary to extract the intermediate frequency,the coarse freqency or the very coarse frequency.

Consider, for example, a system using the MHz fine" data signal for finerange and 1.5 MHz signal for coarse range information. If we assume thatthe fine frequency is combined with coarse frequency by adding bothfrequencies, we have produced a fine frequency (F) and a combinedfrequency (F CS) consisting of the fine frequency and the coarsefrequency (CS).

The phase meter is required to measure the phase of each of the twosignals with respect to a locally generated reference signal and also toresolve the phase information of the received signals to thereby recoverthe true phase of the coarse signal (CS) and the fine frequency (F).Having assumed a combined signal where the fine frequency has been addedto the coarse frequency, we can show that the phase of the detectedcombined signal with respect to a reference signal will be the phase ofthe fine (F) frequency plus the phase of the coarse (CS) frequency,minus the phase of the reference frequency.

In a similar manner, the phase of the detected fine" (F) frequency willbe the phase of the fine frequency minus the phase of the referencesignal. The phase of the coarse signal can then be shown to be the phaseof the combined signal less the phase of the fine signal. The digitalphase counter (comprised of the accumulator, Down and Up counters),after measuring the phase of the fine" frequency, contains the value ofplus fine in the UP counter and minus fine in the DOWN counter. Theinformation in the UP counter is actually the phase of the finefrequency signal and can be immediately read into a register forindicating the fine range.

The output of the DOWN counter, however, is transferred into the UPcounter so as to preset the UP counter with the information presentlycontained in the DOWN counter. This information is actually the negativeof the phase of the fine frequency minus the reference frequency or,expressed positively, is the phase of the reference frequency minus thephase of the fine" frequency.

A subsequent phase measurement of the combined signal with respect to areference signal will generate a signal indicating the phase of the finefrequency plus the coarse" frequency minus the phase of the referencefrequency, minus the phase of the fine signal previously preset into theUP counter. Hence the resulting information will mathematically indicatethe phase of the coarse" signal.

This simultaneous measurement of phase and arithmetic computation haswide applicability in many digital phase meter applications and is notlimited to ranging situations involving coarse" or fine" ranges. Theforegoing example is given by way of explanation only to help explainand understand the concepts of the present invention.

The novel features which are believed to be characteristic of theinvention, both as to organization and method of operation, togetherwith further objects and advantages thereof, will be better understoodfrom the following description considered in connection with theaccompanying drawings in which the preferred embodiment of the inventionis illustrated by way of example. It is to be expressly understood,however, that the drawings are for the purpose of illustration anddescription only and are not intended as a definition of the limits ofthe invention.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram of thepresent invention; and

FIG. 2 is a series of waveforms illustrating the operation of thediagram illustrated in FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG. 1, thereis shown an Accumulator Counter 10 being fed by a Clock Generator 12through an AND gate 14. The AND gate 14 is controlled by a ControlFlip-Flop 16 that also controls a Process Counter 18 which counts thenumber of cycles of the Control Flip-Flop 16 as an indication of thenumber of averages made by the system for any given readmg.

The number of averages is preset and used as a means for stopping thetotal accumulation after the preset number of averages has beenobtained. The output of the Process Counter 18 feeds a Control Logicunit 20 which performs the necessary logical controls and generates thenecessary process commands in response to the needs of externalcircuits.

The Control Flip-Flop 16 is adapted to operate on negative transitionsignals. A Source of Reference Signals 22 is fed to the set terminal ofthe Control Flip- Flop 16 to set or turn ON the Control Flip-Flop in thepresence of a negative going pulse generated by the Source of ReferenceSignals 22.

Similarly, a Source of Data Signals 24 is fed to a reset terminal of theControl Flip-Flop 16 so as to reset or turn OFF the Control Flip-Flop 16in the presence of a negative going signal generated by the Source ofData Signals 24. The Control Flip-Flop 16 will therefore be turned ON bythe Source of Reference Signals 22 and will be turned OFF by the Sourceof Data Signals 24 and hence will allow clock pulses generated by theClock Generator 12 to be accumulated through the AND gate 14 by theAccumulator Counter 10 and the UP/DOWN counters 32 and 30 for a periodof time as measured by the phase difference between the reference signaland the data signal.

In the preferred embodiment, the frequency of the Clock Generator 12 isclosely related to the frequency of the Source of Data Signals 24 so asto generate a thousand pulses for one complete 360 cycle. One thousandaccumulated pulses will therefore indicate a 0 phase difference betweenthe reference signal and the data signal, and similarly, an accumulationof 500 cycles will represent a 180 phase relationship between the datasignal and the reference signal.

The operation of the arithmetic computation will be more fullyappreciated by considering the following situation where the firstreading of the fine frequency (F) is called Data I and represents thephase measurement made as a result of measuring the phase of the finefrequency against the reference signal. The output of the phase metercan be represented as follows:

Phase Data I qbF Ref The subsequent phase measurement identified as DataII will represent the phase measurement between the combined coarsefrequency (CS) and fine" frequency (F) related to the phase of thelocally generated reference (Ref) and can be expressed mathematically asfollows:

Phase Data II F CS Ref Since Data I equals the fine frequency and DataII equals the fine frequency plus a coarse frequency, it is apparenttherefore that the corase frequency can be expressed mathematically asfollows:

Coarse frequency Data II Data I The phase of the coarse frequency or thecoarse frequency itself is therefore determinable by performing thesubtraction of the phase of Data I from the phase of Data II bysubtracting Equation (2) from Equation (1) as follows:

Since in the conventional ranging system the fine frequency phase ismeasured first, it is desirable to rewrite Equation (3) as follows:

Coarse frequency Data I Data II The Accumulator Counter and the UP/DOWNcounters 32 and 30, after measuring the phase of Data I, now contains anaccumulation of pulses equivalent to the phase of the fine frequency.During the measurement process, the output of the Accumulator Counter 10is fed simultaneously to a DOWN Counter 30 and an Up Counter 32. Boththe DOWN Counter 30 and the UP Counter 32 start from zero, and hence theaccumulation in the DOWN Counter 30 will represent a value that is Data1, whereas the accumulation in the UP Counter 32 will represent a valueof Data I or the fine frequency phase itself, as indicated by Equation lThe information in the UP Counter 32 is read out in a Range/Phase Outputregister 34 as an indication of the fine frequency. The register 34 isarranged to readout the range or phase in miles or feet, depending uponthe frequency of the modulating signals used. For the modulatingfrequencies given in this example, the register 34 will have afive-position readout. The three least significant digits are fed by theUP Counter 32 in response to the fine readout.

The DOWN Counter 30 contains Data I, and a transfer of the informationcontained in the DOWN Counter 30 is made into the Up Counter 32, so thatthe UP Counter 32 is now preset with the information from the DOWNCounter 30. A review of Equation (5) shows that the UP Counter 32 nowcontains the information defined as Data I. The Control Logic unit 20generates the necessary process commands for controlling the presettingand resetting of the counters and the readout of the register 34 bymeans of the control line labeled Process Command.

The digital phase meter when measuring the phase difference between theData II signal and the reference signal will accumulate counts in theAccumulator Counter 10 and UP/DOWN Counters 32 and 30 that is equivalentto the phase of fine frequency plus the phase of the coarse frequencyless the phase of the reference as indicated by Equation (2). The outputof the Accumulator Counter 10 is fed to both the DOWN Counter 30 and theUP Counter 32. However, since the UP Counter 32 already contains Data Ias shown by Equation (5 it will be apparent that adding to theaccumulation of Data I an amount equal to Data II will result in anaccumulation in the UP Counter 32 of a number equal to the phase of thecoarse frequency as shown by Equation (5).

The output of the UP Counter 32 is fed to the register 34 in such amanner that the three least significant digits are masked and only thefourth and higher positions are indicated. In this manner the digitsindicating the fine range are not disturbed or modified by any variationbetween the lower order digits of the coarse frequency in view of anydiscrepancy that may exist be tween the total coarse frequencyindication and the fine frequency indication. The information of theData II fed to the DOWN Counter 30 in the last operation is a redundantstep and has no value in this example.

Referring now to FIG. 2, there is shown a series of waveforms in whichcurve (a) represents the reference signal generated by the Source ofReference Signals 22. The Data I signal is indicated in curve (b), andthe output of the Control Flip-Flop 16 is represented by curve (0). Areview of curve (0) will shown that the first negative going transition40 of the reference curve will turn the Control Flip-Flop 16 ON and thatthe first negative going transition 42 of the Data I signal will turnthe Control Flip-Flop 16 OFF, thereby accumulating a count in theAccumulator Counter 10 and UP Counter 32 equivalent to the phasedifference between the reference and Data I signal, which is indicatedby Equation (1).

In a similar manner, curve (d) illustrates an arbitrary signalrepresenting a Data II signal, and curve (e) indicates the phasedifference between the reference signal and the Data II signal as anindication of the time that the Control Flip-Flop 16 is turned ON. Forexample, the negative going transition 40 of the reference (a) turns theControl Flip-Flop 16 ON.

The negative transition 44 of the Data II curve will turn the ControlFlip-Flop 16 OFF as shown in curve (e). It can be appreciated thereforethat the accumulation of pulses as indicated by curve (e) will representa signal indicated by Data II which is a representation of the phase ofthe fine frequency plus the phase of the coarse signal less the phase ofthe reference.

A review of this disclosure will show that the combined signalsrepresenting Data Il may consist of two signals that are either added orsubtracted from each other. The foregoing examples were based on a casewhere the two signals were added. However, the same benefits of thepresent invention may be achieved, where Data [1 is derived bysubtracting the two signals from each other. In such a case Equation (2)will be modified as follows:

Phase Data II qSF CS Ref Similarly, Equation (3) is modified to nowread:

Coarse Frequency Data I Data II The implementation of Equation (7) isvery similar to the implementation of Equation (3) since the Data Iinformation is accumulated in the Accumulator Counter 10, the DOWNCounter 30 and the UP Counter 32, as mentioned previously. Theinformation in the UP Counter 32 is Data I and is transferred to theDOWN Counter 30 which is now preset with the Data I information.

At the same time, the output of the UP Counter 32 is fed to the register34 as before, since the output of the UP Counter 32 is a Data I and thatrepresents the fine frequency phase. In the second operation ofdetermining the phase of the Data II as indicated now by Equation (6),the feeding of the Data II information, via Accumulator Counter 10, intothe DOWN Counter 30 will actually result in Data 11 information beingaccumulated in the DOWN Counter 30 on top of the Data I information.This results in the generation of the coarse phase reading in the DOWNCounter 30, which is then transferred to register 34 for readoutpurposes, in the same fashion as mentioned previously.

While the invention has been described in connec- I tion with a pair ofsignals, it is quite apparent that the process may be expanded foroperation with more than two data phases. It will be apparent,therefore, that the process can be expanded to three or more sequentialdata inputs.

What is claimed as new is: 1. A system for extracting phase informationfrom a pair of CW signals comprising:

means for generating a first signal from a first frequencyrepresentative of a first indicia, means for generating a second signalby algebraically combining said first frequency signal with a signalfrom a second frequency representing a second indicia, means formeasuring the phase difference between said first signal and a localreference signal to obtain a first phase difference indication, meansfor measuring the phase difference between said second signal and saidlocal reference signal to obtain a second phase difference indication,and

means for algebraically combining said first and sec- 6 2. A systemaccording to claim 1 in which said second signal is generated by addingsaid first frequency representative of the first indicia with saidsecond frequency representative of said second indicia.

3. A system according to claim 2 in which said measured first phasedifference and said measured second phase difference are algebraicallydifferenced for obtaining the phase of said second frequency.

4. A system according to claim 2 which includes means for accumulatingsaid first phase difference simultaneously in a DOWN counter and an UPcounter,

means for reading the output of said UP counter as an indication of saidfirst indicia, and

means for subsequently presetting the information from said DOWN counterinto said UP counter,

means for accumulating said second phase difference in said UP counter,and

means for reading the output of said UP counter as an indication of saidsecond indicia.

5. A system for extracting ranging information from a pair of modulatingsignals comprising:

means for generating a first modulating signal from a first frequencyrepresentative of fine range,

means for generating a second modulating signal by algebraicallycombining said first frequency with said second frequency representativeof corase range, means for measuring the phase difference between saidfirst modulating signal and a local reference signal to obtain a firstphase difference indication,

means for measuring the phase difference between said second modulatingsignal and said local reference signal to obtain a second phasedifference indication,

and mean for algebraically combining said first and second phasedifference indication with each other to generate the phase of saidsecond frequency representative of corase range.

6. A system according to claim 5 in which said second modulating signalis generate by subtracting said first frequency representative of finerange from said second frequency representative of coarse range.

7. A system according to claim 6 which includes means for accumulatingsaid first phase difference simultaneously in :1 DOWN counter and an UPcounter,

means for reading the output of said UP counter as an indication of finerange,

means for subsequently presetting the information from said UP counterinto said DOWN counter, means for accumulating said second phasedifference in said DOWN counter and,

means for reading the output of said DOWN counter as an indication ofcoarse range.

8. A system for extracting ranging information from a pair of modulatingsignals comprising:

the first signal having a known characteristic of range adapted tomodulate a carrier signal,

the second signal having a combination of said known characteristic andan unknown characteristic of range adapted to modulate said carriersignal,

a digital phase detector for measuring the phase of said first signalrelative to the phase of a known reference signal,

a digital phase detector for measuring the phase of the first signalrelative to the phase of the known reference signal,

said digital phase detector measuring the phase of the second signalrelative to the phase of said known reference signal, and

means for algebraically combining said measured phase differences in theproper sense to obtain an output representing coarse range.

means for generating a first modulating signal from a first frequencyrepresentative of fine range,

"UNITED. STATES PATENT omcr 7 CERTIFICATE OF CORRECTION Patent N0, Dat dOctober 16,

Inven o g RICHARD E; WATT It is certified that error appears in theabove-identified patent and that said Letters Patent are herebycorrected as shown below:

Col. .6, line 47 change "shown" to --showli e 43, change "generate"to.generated-- Col. 10, lines 10-11, delete "means for generating afirst modulating signal from a first frequency representative of fine INTHE DRAWINGS: I

In Fig. the third line, cancel "on" and change the ex pression wf to'--F-- On the last line, cancel "off". and change the expression "f to FI I Signed and sealed this 3rd day of December 1974'.

. a a a Attest: McCOY M. GIBSON JR. 0.. MARSHALL DANN Arresting OfficerCommissioner of Patents S GUVERNMENT PRINTING OFFICE: (959 O-JEG'SJ-i iUN TED. STATES PATENT OF ICE CERTIFICATE OF CORRECTION Patent 3,766,555Dated October 16, 1973'- Inventor(s) RICHARD H. WATT It is certifiedthat error appears in the above-identified patent and that said LettersPatent are hereby corrected as shown below:

Col. .6, lineh47 I change "shown" to show-- Col. 10, lines 10-11, delete"means for generating a first modulating signal from a first IN HEDRAWINGS:

In 2: 5 in the third line, cancel "on" and change the aex pressioniwf to--F- On the last line, cancel "off". and change the expression "f to -F-I g Signed and sealed this 31m day of December 1974.

' (TE'EAL) 5 Attest: I V V McCOY M; GIBSON JR. c. MARSHALL DANNAttesting Officer I Commissioner of Patents uscowv-De sows-P69 FORMPO-iOSO (10-61;

1 u s cuvsmmsm' PRINTING OFFICE use o-aas-sa:

Col. 8, line 43, change "generate" tcgenerated- I frequencyrepresentative of fine i l g n. I I

1. A system for extracting phase information from a pair of CW signalscomprising: means for generating a first signal from a first frequencyrepresentative of a first indicia, means for generating a second signalby algebraically combining said first frequency signal with a signalfrom a second frequency representing a second indicia, means formeasuring the phase difference between said first signal and a localreference signal to obtain a first phase difference indication, meansfor measuring the phase difference between said second signal and saidlocal reference signal to obtain a second phase difference indication,and means for algebraically combining said first and second phasedifference indications with each other to generate the phase of thesecond frequency representative of said second indicia.
 2. A systemaccording to claim 1 in which said second signal is generated by addingsaid first frequency representative of the first indicia with saidsecond frequency representative of said second indicia.
 3. A systemaccording to claim 2 in which said measured first phase difference andsaid measured second phase difference are algebraically differenced forobtaining the phase of said second frequency.
 4. A system according toclaim 2 which includes means for accumulating said first phasedifference simultaneously in a DOWN counter and an UP counter, means forreading the output of said UP counter as an indication of said firstindicia, and means for subsequently presetting the information from saidDOWN counter into said UP counter, means for accumulating said secondphase difference in said UP counter, and means for reading the output ofsaid UP counter as an indication of said second indicia.
 5. A system forextracting ranging information from a pair of modulating signalscomprising: MEANS FOR GENERATING A FIRST MODULATING SIGNAL FROM A FIRSTFREQUENCY REPRESENTATIVE OF FINE RANGE, means for generating a secondmodulating signal by algebraically combining said first frequency withsaid second frequency representative of corase range, means formeasuring the phase difference between said first modulating signal anda local reference signal to obtain a first phase difference indication,means for measuring the phase difference between said second modulatingsignal and said local reference signal to obtain a second phasedifference indication, and mean for algebraically combining said firstand second phase difference indication with each other to generate thephase of said second frequency representative of corase range.
 6. Asystem according to claim 5 in which said second modulating signal isgenerate by subtracting said first frequency representative of finerange from said second frequency representative of coarse range.
 7. Asystem according to claim 6 which includes means for accumulating saidfirst phase difference simultaneously in a DOWN counter and an UPcounter, means for reading the output of said UP counter as anindication of fine range, means for subsequently presetting theinformation from said UP counter into said DOWN counter, means foraccumulating said second phase difference in said DOWN counter and,means for reading the output of said DOWN counter as an indication ofcoarse range.
 8. A system for extracting ranging information from a pairof modulating signals comprising: the first signal having a knowncharacteristic of range adapted to modulate a carrier signal, the secondsignal having a combination of said known characteristic and an unknowncharacteristic oF range adapted to modulate said carrier signal, adigital phase detector for measuring the phase of said first signalrelative to the phase of a known reference signal, said digital phasedetector sequentially measuring the phase of second signal relative tothe phase of said known reference signal, and means for combining saidmeasured phase differences in the proper sense for obtaining an outputrepresenting said unknown characteristic.
 9. In a system for extractingranging information from a pair of modulating signals in which the firstsignal has a known characteristic of fine range and the second signal isan algebraic combination of said first signal and a second signal havingunknown characteristic of coarse range the improvement comrpising: adigital phase detector for measuring the phase of the first signalrelative to the phase of the known reference signal, said digital phasedetector measuring the phase of the second signal relative to the phaseof said known reference signal, and means for algebraically combiningsaid measured phase differences in the proper sense to obtain an outputrepresenting coarse range. means for generating a first modulatingsignal from a first frequency representative of fine range,